Measurement of frequency response with improved sensitivity and accuracy

Abstract

A testing system including a swept frequency signal generator to apply a signal to a device under test, a broadband detector for receiving and detecting that signal returned from the device under test, a cathode ray tube connected to display the detector output level as a function of frequency applied to the device under test after high gain low frequency amplification of the detector output signal. The detector input is provided with an input impedance across its two conductors that is matched to the impedance of the circuit under test, with a capacitor connected subsequent to the load impedance in each of the two detector leads. The detector circuit between the capacitors and an input to the high gain amplifier is unconnected to ground except at one of the amplifier input terminals, thereby to improve the accuracy and sensitivity of measurements. Any nonlinear frequency response of the detector are corrected by adding to the measured signal before display a voltage proportional to the frequency of the signal generator output. In order to reduce error from amplifier drift, the high gain amplifier is re-zeroed during the retrace of the signal generator. In order to make the cathod ray tube display more easily readable, frequency markers are superimposed upon the display signal by an electronic circuit that generates marks of varying precise widths at varying frequencies that are stacked together to provide composite marks of controllable width and discernable height.